The regretably common disease atherosclerosis, which is a type of arteriosclerosis, is characterized by the formation of atherosclerotic lesions (also known as atherosclerotic plaques) on the inner wall of the aorta and the large and medium-sized arteries. The most important symptom in the early stages of this disease is hypertension. If uncorrected, however, the disease can lead to total blood vessel blockage, and ultimately, death of the patient. The atherosclerotic lesions are masses of fatty material associated with fibrous connective tissue, very often with secondary deposits of calcium salts and blood constituents. Human atherosclerotic lesions are characterized by a large lipid content, which may account for as much as 60 percent of the dry weight of some advanced lesions. Three major classes of lipids are found, i.e. cholesterol, cholesterol esters and phospholipids.
One surgical technique currently practiced for correcting problems arising from arteriosclerotic lesions is coronary or peripheral arterial bypass surgery, in which a blood vessel segment removed from another part of the patient's body, e.g. a saphenous vein segment, or a synthetic vascular graft is implanted in parallel with the occluded artery. Although arterial bypass surgery has been practiced with great success for many years, it is a major surgical operation with inevitable attendant risks and the medical profession is therefore continuously searching for more conservative techniques for reducing vascular obstructions such as arteriosclerotic lesions without bypass surgery.
Another surgical technique currently practiced with considerable success in the treatment of arteriosclerosis is transluminal angioplasty, in which a balloon catheter is inserted into an affected blood vessel and the balloon then expanded outwardly against the occlusion to recannulate the vessel. One disadvantage of this technique is that it cannot be employed when the vessel is already fully blocked (or almost so) by occlusions. Also, its practice results principally in redistribution (i.e. compaction) rather than physical or chemical removal of the lesion material, most of which remains in the affected blood vessel wall and can serve as a site for future occlusive growth.
Recently it has been proposed to reduce vascular occlusions such as arteriosclerotic lesions by the practice of laser revascularization angioplasty, in which electromagnetic energy generated by a laser is carried by one or more optical fibers to the vicinity of the occlusion and directed at the occlusion. Uptake of the laser energy by occlusion material results in its conversion to relatively low molecular weight organic substances, which are dissolved into and carried away by the blood stream. Suitable apparatus for the practice of laser revascularization angioplasty are disclosed in U.S. Pat. No. 4,207,874; U.S. Pat. No. 4,418,688; World Published Patent Application 8301893, published Jun. 9, 1983; World Published Patent Application 8303188, published Sep. 29, 1983 and World Published Patent Application 8302885, published Sep. 1, 1983. A highly significant advantage of laser revascularization angioplasty is that its practice can result in the essentially complete removal of a vascular occlusion (e.g. an arteriosclerotic lesion) in a surgical procedure that is far less invasive than bypass surgery. However, because of the difficulty in designing a laser catheter system whose use assures that the laser beam is carefully directed to impinge only upon the undesired occlusion, the practice of this technique involves a risk of damage to blood constituents and healthy surrounding tissues, particularly the surrounding non-arteriosclerotic blood vessel tissue.